Given the widespread deployment of Wireless Broadband systems in the recent years and the increasing requirements of multimedia applications, the need for high capacity and enhanced reliability has become imperative. Multiple-Input Multiple-Output (MIMO) technology, which uses multiple antennas for emitting and receiving, and its single receiving antenna version, Multiple-Input Single-Output (MISO), promise a significant performance boost and have been incorporated in the emerging Broadband wireless systems.
MIMO/MISO schemes can be used to establish reliable point-to-point communications with a high throughput (single user MIMO/MISO), but, by using multiple antennas at the transmitter, it is also possible to achieve simultaneous point-to-multipoint transmissions and serve multiple users at the same time, through the same frequency and code. The MIMO/MISO multiuser transmission concept, where data streams are assigned to different users, is shown to increase the overall system capacity when compared to single-user MIMO transmission where all streams are dedicated to just one user.
In order to implement multiuser MIMO/SIMO systems, a Medium Access Control (MAC) Protocol has to be established. The most important operations that must be handled by a MAC protocol are the channel access, the scheduling and the feedback acquisition.
U.S. 2009/0046681 A1 provides a very interesting approach to make the MAC for more than one stream at the same time, where this can be applied to MIMO point-to-point spatial multiplexing and/or to OFDM subcarriers allocation. The idea is to enlarge the headers to support the channel estimation of more than one channel at the same time. It does not make any consideration of multiuser MIMO, where the data streams are for different users in the same time, frequency and code; where each user receives its signal together with the other user signal, and no collision is decided in the system.
Other contributions to the field, include KR 2009/0004662, in which the users selection in MIMO multiuser systems is presented, where the selection is based on the SNIR metric. EP 1,882,313 is related to the receiver side and whether to decide that several users access the channel at the same time, as the receiver with its multiuser detection capabilities (MUD) can separate the different users or not. All the intelligence is located at the receiver side and it does not deal with users scheduling, MAC design nor the transmitter decisions.
Finally, in WO 2008/012766 the considered scenario is with one base station and several users in the network, and how to manage the access of the users through OFDMA or any other multicarrier system. MC-CDMA for example is proposed, but no more than one user can be serviced through the same time, frequency and code, i.e. no spatial multiplexing is presented in the system.
Finally, a practical MAC scheme named Multi-User Distributed Coordination Function (MU-DCF), intended for multiuser MIMO/MISO Ad Hoc transmissions, has been presented in “A MAC Protocol with Multi-user MIMO Support for Ad-Hoc WLANs” (J. Mirkovic, J. Zhao, and D. Denteneer, IEEE PIMRC, September 2007). This protocol is a modification of the 802.11 DCF where a four-way handshake is initiated by a special multiuser Ready To Send (RTS) frame that includes a polling address list. Nevertheless, the inclusion of the polling list in the RTS frame uses part of the network resources, thus resulting in a reduction of the available throughput.